A controller area network (CAN) bus, such as that compliant with the ISO 11898 standard, is used in several systems including industrial, automotive, robotic, and motor control systems to provide a serial communication physical layer. The robust CAN bus provides low power requirements, space savings and reduced resources. As shown in FIG. 1, a CAN node is comprised of three basic parts: a processor, a network controller, and a transceiver. The transceiver interfaces the single-ended CAN controller with the differential CAN bus. The bus, as shown in FIG. 2, includes multiple nodes that transmit messages on-demand by any node whenever the bus is free. The transceiver broadcasts data such that all nodes receive each message sent on the bus, including the node that sent it. The effect of broadcasting of data allows multiple nodes to utilize the data transmitted.
Texas Instruments has introduced CAN transceiver models: SN65HVD230, SN65HVD231, and SN65HVD232, for use in applications employing the CAN serial communication physical layer in compliance with the ISO 11898 Standard. As a CAN transceiver, as shown in FIG. 3, the disclosed model provides differential transmit capability to the bus and differential receive capability to a CAN controller at speeds of up to 1 Mbps. Designed for operation in especially harsh environments, the CAN transceiver features cross-wire, over voltage and loss of ground, over-temperature protection as well as −20 V to 20 V common-mode range, and withstands transients of ±30 V.
Specifically, a controller area network is a differential signaling system used for communication between modules in automobiles that makes use of two bus lines namely, CANH and CANL. Data communication occurs mainly by differentiating between the two possible states on the bus—a dominant state during which a differential voltage Vdiff is established between the two bus lines, CANH and CANL, and a recessive state during which there is no differential voltage as is shown in the graphic representation of FIG. 4. In the latter state, bus lines, CANH and CANL are both in a high-impedance state and the driver is inactive, while the receiver is still able to receive data.
The dominant and recessive states are used to signal binary data which is detected by the receiver. In operation, the receiver simply signals a logic high when the differential voltage on the CAN bus is more than 900 mV and signals a logic low when the differential voltage is less than 500 mV. The input differential voltage can vary between 0 and 3V while the common mode range for the input signal can vary from −20 to 20V. This voltage range is substantially greater than the supply voltage of 5V. The threshold voltage where the receiver switches state from low to high is approximately 800 mV (VH) and the lower threshold voltage when the receiver switches state from high to low is approximately 600 mV (VL). The hysteresis (VH−VL) should be greater than 100 mV, while the thresholds should be in the 900 mV-500 mV window.
The time from when the differential voltage of 900 mV is established on the bus to the time when the data is output from the receiver is critical since it directly effects loop time which is a specification that indicates the speed of the transceiver. Loop time is the time delay between the instant when the input voltage to the receiver reaches 50% of the final (logic high or low) voltage during a rising or a falling transition to the instant when the output of the receiver changes by 50% during a rising or falling transition. Accordingly, delay in the receiver affects the speed of the transceiver. Therefore, the receiver must be fast. In addition, the receiver should also have extremely good common mode rejection for better immunity against EMI interference. EMI creates errors in reading the data. In order to avoid these errors, the data rate must be slowed so that the data sampling time can be increased. The reduced data rate increases the latency of devices connected to the CAN bus or reduces the number of devices the bus can handle within a specified latency.
There is a need for a CAN receiver architecture design that provides better immunity against EMI interference than conventional designs. In addition, this CAN receiver architecture must be faster than conventional designs and possess an improved common-mode rejection, while operating over a wide input common mode range.
It is a general object to overcome, or at least reduce the effects of one or more of the problems set forth above.
This and other objects and features of the invention are provided, in accordance with one aspect of the intention by a Controller Area Network (CAN) receiver comprising a voltage divider coupleable to a controller area network bus for dividing a signal on the bus by a predetermined factor N to generate an input signal. A front end amplifier amplifies the input signal by a predetermined factor substantially equal to 1/N.
Another aspect of the invention includes a method of receiving signals on a Controller Area Network (CAN) bus comprising dividing a bus signal by a predetermined factor N to generate an input signal. The input signal is amplified by a predetermined factor substantially equal to 1/N.
The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed.